Substitution reactions of benzo[b]thiophen derivatives. Part IV. Nitration of 3-bromo-2-methylbenzo[b]thiophen and 2,3-dimethylbenzo[b]thiophen

Abstract

Nitration of 3-bromo-2-methylbenzo [b]thiophen in a mixture of sulphuric and acetic acids gives 2-methyl-3-nitrobenzo[b]thiophen (44%), 3-bromo-2-methyl-6-nitrobenzo[b]thiophen (31%), and 3-bromo-2-methyl-4-nitrobenzo[b]thiophen (22%).

The nitration of 2,3-dimethylbenzo[b]thiophen in acetic acid is re-examined and the major products are shown to be 2,3-dimethyl-6-nitrobenzo[b]thiophen, 3-methyl-2-nitromethylbenzo[b]thiophen, 3-methylbenzo[b]thiophen-2-carbaldehyde, and 2,3-dimethyl-4-nitrobenzo[b]thiophen. The last compound was not reported in an earlier study by Bordwell and Cutshall. A minor product from the nitration reaction is now shown to be a bis-(o-substituted phenyl) disulphide (1), and its formation is rationalised in terms of a ring-opening reaction. The synthesis of a model compound, 3-nitro-3-phenylbutan-2-one, is reported and an attempt to synthesise the disulphide (1) is described.